In recent years, the development of a variety of electronic equipment in accordance with high-speed digital interface standards, such as the HDMI (High Definition Multimedia Interface) standard or DVI (Digital Visual Interface) standard, is in progress. The HDMI standard, in particular, defines the transmission of an audio signal and a control signal during the video blanking interval. These high-speed digital interface standards adopt the so-called TMDS (Transmission Minimized Differential Signaling) serial transmission system.
FIG. 6 is a diagram that shows an example of a conventional DVI-compliant signal transmission system (DDWG, “Digital Visual Interface,” Revision 1.0, Apr. 2, 1999).
The signal transmission system of FIG. 6 includes TMDS encoder/serializers 2601, 2602, 2603 on the transmission side and TMDS decoder/recoveries 2604, 2605, 2606 on the reception side.
On the transmission side, an 8-bit red component signal R, a data enable signal DE, and 2-bit control signals CTL2, CTL3 are input to the TMDS encoder/serializer 2601. Also, an 8-bit green component signal G, a data enable signal DE, and 2-bit control signals CTL0, CTL1 are input to the TMDS encoder/serialzer 2602. Furthermore, an 8-bit blue component signal B, a data enable signal DE, and 2-bit synchronization signals HSYNC, VSYNC are input to the TMDS encoder/serializer 2603.
The TMDS encoder/serializer 2601 encodes the 8-bit red component signal R according to TMDS to convert it into a 10-bit, serial signal. The purpose of the 8- to 10-bit conversion is to transform the signal into a format suitable for high-speed transmission by decreasing the change points of data.
The TMDS encoder/serializer 2601 also encodes the 2-bit control signals CTL2, CTL3 to convert them into a 10-bit, serial signal. It further encodes the data enable signal DE to convert it into a serial signal.
In this manner, the TMDS encoder/serializer 2601 sends the serial signal containing the time-division multiplexed red component signal R, data enable signal DE, and control signals CTL2, CTL3 out to the transmission line of a channel ch2.
Similarly, the TMDS encoder/serializer 2602 sends the serial signal containing the time-division multiplexed green component signal G, data enable signal DE, and control signals CTL0, CTL1 out to a channel ch1 as a transmission line. The TMDS encoder/serializer 2603 also sends the serial signal containing the time-division multiplexed blue component signal B, data enable signal DE, and synchronization signals HSYC, VSYNC out to the transmission line of a channel ch0.
On the reception side, the TMDS decoder/recovery 2604 decodes the serial signal sent via the transmission line of the channel ch2 according to TMDS to recover the 8-bit red component signal R, data enable signal DE, and 2-bit control signals CTL2, CTL3. The TMDS decoder-recovery 2605 decodes the serial signal sent via the transmission line of the channel ch1 according to TMDS to recover the 8-bit green component signal G, data enable signal DE, and 2-bit control signals CTL0, CTL1. The TMDS decoder/recovery 2606 decodes the serial signal sent via the transmission line of the channel ch0 according to TMDS to recover the 8-bit blue component signal B, data enable signal DE, and 2-bit synchronization signals HSYNC, VSYNC.
The data enable signals DE, respectively, indicate the periods of time in which the red component signal R, green component signal G, and blue component signal B are present, each attaining a high level or “H” level in its active state.
As an example, the period in which the data enable signal DE attains low level or “L” level corresponds to the period of a horizontal synchronization signal or the period of a vertical synchronization signal for video. Note also that the above-mentioned four control signals CTL0, CTL1, CTL2, CTL3 stand ready, although the current DV1 standard leaves the control signals CTL0, CTL1, CTL2, CTL3 unused. More specifically, the control signals CTL0, CTL1, CTL2, CTL3 remain at zero all the time.
The HDMI standard, on the other hand, provides the transmission of a digital audio signal in addition to a digital video signal defined by the DV1 standard. The transmission of the digital audio signal may generate audio noise due to cable disconnection or plugging in, transmission error, power-supply ON/OFF, or switching of video/audio formats, which gives a user a disagreeable feeling or damages the speaker.